Yes, it is very likely that you may put on weight if you revert back to a sedentary lifestyle after discontinuing exercise. When you begin exercise, your fat mass gets converted to muscle mass which increases your body's metabolism. When you stop exercising, muscles get converted back to fat, slowng down your metabolism as fat is metabolically inactive tissue. If you begin exercise, make sure you are persistent and continue being physically active even if you are not going to a gym. You can stay active by playing outdoor games like badminton, football or cricket. Walk as much as possible as it helps in raising your metabolism.